Many strategies for the conversion of solar energy into chemical bonds involve electrocatalytic (or photocatalytic) cleavage of water to form hydrogen and oxygen. The reducing equivalent, H2, is thus an energy carrier because it can be re-oxidized, either by combustion to give heat or catalytically in a fuel cell to give electricity. There is a disabling problem with large-scale utilization of hydrogen as a fuel, since it is a gas under ambient conditions, thus limiting its volume energy density (0.013 MJ l−1). As a result, physical methods based hydrogen storage technologies (compression, cryogenic liquefaction, adsorption) involve low capacity, high costs or safety issues [1]. Therefore, the discovery of highly weight-efficient strategies for on-demand hydrogen release from hydrogen-rich liquids has value. Formic acid (HCO2H, FA, 7.5 MJ l−1) is a hydrogen carrier, owing to its ability to release hydrogen under mild conditions with only CO2 as a by-product [1-6], which can then be recycled, in principle, to give a carbon-neutral fuel cycle [7-9].
To date, many efficient heterogeneous [10-22] and homogeneous [9, 23-38] catalysts for FA dehydrogenation have been developed. Heterogeneous catalysts have advantages of separability and reusability [11], while homogeneous catalysts are generally more efficient. The best turnover numbers (TONs) achieved in homogeneous catalysis are (1) 41M, by a catalyst system composed of [RuCl2(benzene)]2, the ligand diphenylphosphinoethane and a FA/Et3N adduct as substrate developed by Boddien et al. 9 and (2) 983,642, by a system composed of an iron pincer complex and LiBF4 developed by Bielinski et al. [35]. The highest turnover frequency achieved is 228,000 h_1 by an iridium catalyst developed by Hull et al. [27]. In heterogeneous catalysis, the highest TOF achieved is 7,256 h_1, by palladium nanoparticles immobilized on carbon nanospheres developed by Zhu et al. 22. Also, homogeneous catalysts generally are more selective, producing less carbon monoxide, a common byproduct of FA dehydrogenation. This is essential, because CO is a fuel cell catalyst poison. Still, no known system is stable and reactive through multiple uses, air and water tolerant, selective against CO formation, and functions in neat FA liquid. Each of these is critical to achieving a usable hydrogen generation system based on FA.
Accordingly, there is a need for improved catalyst systems for the dehydrogenation of formic acid.